1. Field of the Invention
This invention relates to a semiconductor device for use as a high-speed switching device and a power device, and relates, for example, to a horizontal-type field effect transistor.
2. Description of the Related Art
Heretofore, a power IC has been known in which there are formed, on the same substrate, a high withstand voltage semiconductor element for use in, for example, a high withstand voltage drive circuit, and a low withstand voltage semiconductor element for use in, for example, a low withstand voltage drive circuit. Many uses have been conceived regarding the power IC. Low on-resistance is required in a MOS field effect transistor having a high withstand voltage structure (hereinafter, a high voltage MOSFET) for use in an output stage of the power IC of this kind. To achieve the low on-resistance, the high voltage MOSFET is generally manufactured by a micro-process.
Meanwhile, as progress has been made in miniaturization and the high voltage MOSFET has come to be manufactured under the rule of 0.25 μm or less, an element separation region is changed from LOCOS to shallow trench isolation (STI). A high voltage MOSFET having a structure in which an element separation region is formed by the STI is described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-37267. In the high voltage MOSFET having such a structure, for example, in an element of a withstand voltage 20 V system, there may occur a problem that a source-drain voltage snaps back at about 15 V when a gate is on.